Leave the leeks to soak for 15 minutes in the buttermilk. Meanwhile, mix together the flour, cornmeal, and salt. Fill a cast iron skillet with 1 inch of canola oil, and heat it to 375°F.

Allow the excess buttermilk to drip away, and dredge the leeks, individually, in the flour mixture. Fry in small batches until crisp and golden brown, from 1 to 3 minutes. Drain on paper towels, and sprinkle with extra salt.